The seat of the Mantua bishop holds many secrets within its walls.

This massive blockhead sculpture hides an entire library.

A patchwork of ruins from various civilizations in one of the world's oldest continually inhabited cities.

Inmates dig the graves at the largest prison cemetery in the U.S., the final resting place of both cowboys and Indians.

An ancient Phoenician citadel once stood at the site where Queen Dido first landed, according to legend.

The ancient heart of Tunis is an enormous maze of narrow streets and alleyways.

The best-preserved Roman small town in North Africa.

An underground city of salt.

Vibrant street art created with mixed media.

Of the many chalices in the running for the Holy Grail, this one in Valencia Cathedral sure looks the part.

This amazing example of Valencian Art Nouveau looks more like a cathedral than a public market.

Khmer Rouge's high security prison.

The only quarter of the Old City not dedicated to a major world religion.

A "miracle" that seems to produce more holy fistfights than holy revelations.

Descend 2,030 years into the past in an ancient tunnel built to support Jerusalem's most famous wall.

Dozens of elegant ceramic tiles decorate these ancient walls, each displaying the Lord's Prayer in a different language.

The castle where Martin Luther secretly translated the New Testament from Greek to German while in hiding.

The largest wine cellar in the world is a 150-mile network of underground caves near the capital of Moldova.

This 13th century Orthodox monastery is built right into the Moldovan cliffside.

Azerbaijani ghost town, decaying after a war left it an armed forces buffer zone.

A spectacular Petrine Baroque summer palace commissioned for Catherine I by Peter the Great.

A massive collection of petroglyphs dating back some 40,000 years.

Resonant stone, that has been played since prehistoric times.

Home to nearly a third of the world's mud volcanoes, Azerbaijan is home to a messy, bubbling, and sometimes explosive landscape.